THanks to Oliver, I am now in possession of a set of 6.0.8 disks (4x800Kb).
I booted with Disk 2 and then selected the "Apple HD SC Setup" app. I then select "Initialize" with my HD selected ("SCSI Device:4"), but this fails with a message: "Sorry, a system error occurred - address error"

I also tried to re-partition the same HD with the Maximum Partition option, but that also fails with the message "Customizing the partitions failed. THe partition map's structure is invalid"

Any clue greatly appreciated! Also if anybody has a link to the Plus or SE manuals (PDF), that would also be great

I don't think it's broken, it does boot and seems to operate properly, but I think I might be doing something. When you re-installed yours, did you also boot with disk2, and then run the same app as me?